How do I change the track object of an Edit Object actuator?
Edit Object Actuator
The way you change an Edit Object actuator's object to track is with object.
Here is what the if statement section of your code looks like.
if sens.positive:
    actu.object = sens.hitObject
    cont.activate(actu)


Steering Actuator
The way you change the Steering Actuator's object to track is with target.
Here is what the if statement section of your code looks like.
if sens.positive:
    actu.target = sens.hitObject
    cont.activate(actu)


I initially thought you were using a different actuator (Steering) that is why both methods are listed.